ssh-add gives the error message:
Could not open a connection to your authentication agent, this is because your
ssh-agent has stopped running for some reason and is unable to restart automatically. Try removing the ssh environment file.
For example on PUMA:
puma$ rm ~/.ssh/environment.puma
Then log out and back in again. You should then see a message similar to:
Initialising new SSH agent...
And you should then be able to run